Two families of internal-chip deep hole drilling dominate production bores above about 18 mm. BTA (Single Tube System, STS) seals against the workpiece with a pressure head, forcing coolant down the annulus and chips back through the drill tube center — at 5–10× the feed of gundrilling. Ejector drilling (Double Tube System, DTS), built by Sandvik on the same head geometry, replaces the face seal with Venturi suction inside a concentric tube pair, trading some chip-clearance efficiency for the freedom to run on conventional lathes and machining centers.
BTA and ejector drilling are not competing inventions — the ejector system is a variation of BTA developed by Sandvik so that internal-chip deep hole drilling could run on machine tools that were never built for it. Both use a multi-edge drill head with carbide guide pads, both evacuate chips inside the tool rather than through a V-groove on the outside, and both cut continuously with no peck cycles. The difference is entirely in how coolant and chips are routed and how the high-pressure coolant is contained.
| Attribute | BTA (Single Tube System, STS) | Ejector (Double Tube System, DTS) |
|---|---|---|
| Tube construction | One round tube — no V-groove, maximum torsional rigidity | Inner + outer concentric tubes |
| Coolant path | Annulus between tube O.D. and hole wall | Annulus between inner and outer tubes |
| Chip path | Through center of the single tube, pressure flushed | Through the inner tube, Venturi suction |
| Face seal | Pressure head (BOZA) seals against the workpiece | None — a guide bush is all that is needed |
| Chip-clearance zone | > 60% of the hole area | 35–40% of the hole area |
| Machine | Dedicated BTA deep hole machine | Conventional lathe, machining center, boring mill |
| Origin | Boring & Trepanning Association standard | Sandvik adaptation of the BTA system |
Sources: Baucor “Deep Hole Drills”; ISCAR “Deep Hole Drilling Index”; Sandvik Deep Hole Machining handbook; CTE “Three Deep-Hole Drilling Systems”.
BTA seals the bore entrance so coolant cannot escape. A pressure head (BOZA) is clamped against the workpiece face and carries the drill bushing; coolant is injected under pressure into the annular gap between the round drill tube outer wall and the machined hole wall, sweeping past the cutting edges and guide pads and carrying chips into the tube center for evacuation.
The pressure head (BOZA) clamps against a flat workpiece face, containing coolant up to 100+ bar. The face must be square to the spindle — an off-square start is a leading cause of bore drift.
High-pressure coolant (typically 15–100 bar; minimum ~225–250 psi, rising past 1,400 psi with depth) flows down the annulus between tube O.D. and hole wall.
The multi-edge head (brazed, spade, or indexable inserts) removes stock while carbide guide pads — main pad near 178°, secondary near 276° — steer the head and burnish the bore.
Chips and spent coolant are pushed through the center of the tube and out through the machine spindle — chips never touch the machined surface.
The ejector drill is a concentric pair of tubes. Coolant is pumped into the annulus between the outer and inner tubes. Roughly 60–70% of it is deflected through Venturi slots machined near the drill head, creating a localized low-pressure zone that generates suction; the remaining 30–40% exits at the head to cool and lubricate the cutting edges. Chips and spent coolant are then drawn back through the inner tube by that suction.
High-volume, moderate-pressure coolant (10–50 bar) is pumped into the annulus between the outer and inner tubes — entirely inside the drill, never against the part.
~60–70% of the flow passes through ejector slots in the inner tube, converting pressure energy to velocity and creating the suction (the Venturi / ejector effect).
The remaining 30–40% is directed onto the cutting edges and guide pads — the same multi-edge head family as BTA.
Chips are drawn through the inner tube under the Venturi vacuum and out of the machine. Only a guide bush (or a pre-drilled pilot hole) guides the entry.
| Parameter | BTA / STS | Ejector / DTS |
|---|---|---|
| Diameter range | ~20–700 mm (ISCAR catalog 14.5–246 mm; AGrade 20–500 mm) | 18–200 mm (ISCAR 18.4–169 mm; AGrade 18–150 mm) |
| Practical minimum diameter | ~19–20 mm | ~18 mm — double-tube construction leaves no chip space below this |
| Max depth | Over 100×D in production | ~100×D; ~1,000 mm standard, ~2,000 mm with special connectors |
| Chip-clearance zone | > 60% of hole area | 35–40% of hole area |
| Coolant pressure | 15–100 bar (min ~225 psi; can exceed 1,400 psi with depth) | 10–50 bar — driven by flow rate, not static pressure |
| Coolant flow | 50–500+ L/min depending on diameter | High volume at moderate pressure (30–120 L/min typical) |
| Workpiece face seal | Required — pressure head (BOZA) with drill bushing | Not required — only a guide bush or pilot hole |
| Retrofit on existing machines | No — dedicated BTA machine required | Yes — lathes, turning centers, machining centers, boring mills |
| Productivity vs gundrilling | 5–10× feed rate (typically 5–7×) | 3–5× feed rate |
| Typical tolerance | ±0.05 mm / IT9 | ±0.04 mm / IT9–IT10 |
| Chip evacuation mechanism | High-pressure coolant flush | Venturi suction |
| Filtration requirement | 50 μm minimum; 20 μm recommended for guide pad life | 50 μm adequate — larger coolant passages |
Both systems need horsepower and serious coolant infrastructure. The difference is where that infrastructure lives.
| Requirement | BTA / STS | Ejector / DTS |
|---|---|---|
| Machine type | Dedicated BTA deep hole machine with pressure head (BOZA) | Retrofit kit on lathe, turning center, machining center, or boring mill |
| Spindle power | ~3.2 kW per 25 mm of bore (11 hp/inch) | High-horsepower host spindle — retrofit does not remove the power demand |
| Coolant system | High pressure (15–100 bar) and high flow (50–500+ L/min) | High flow at moderate pressure (10–50 bar); pump upgrades are common |
| Spindle bore | Must pass the drill tube plus returning chips | Must pass the outer tube — typically 1.5–2× the drill diameter |
| Entry guidance | Drill bushing mounted in the pressure head | Guide bush, or a pre-drilled pilot hole can substitute |
| Whip / steady support | Tube support per machine design | Support the outer tube every 40–60×D |
| Filtration | 50 μm minimum; 20 μm recommended | 50 μm adequate |
| Fire / mist | Mist extraction + spark suppression (oil coolant) | Same — high-volume oil mist is generated either way |
The capital decision usually settles the method: a dedicated BTA machine is a major purchase, while an ejector retrofit rides on iron the shop already owns.
| Cost Driver | BTA / STS | Ejector / DTS |
|---|---|---|
| Capital | Dedicated machine: new $60k–high six figures; used $20k–$100k+ | Retrofit kit, rotary connector, pump on an existing machine |
| Coolant system | High-pressure, high-volume pumps and filtration | High-flow moderate-pressure pump; upgrade is a frequent line item |
| Setup per hole | Pressure head + flat face prep + bushing | Guide bush or pilot hole — faster changeover |
| Tooling | Brazed / spade / indexable multi-edge heads, regrindable | Same head family plus the Venturi nozzle ring |
| Per-hole cost, low volume | Higher — amortizing a dedicated machine over few holes | Lower — leverages existing machine time |
| Per-hole cost, high volume | Lowest — 5–10× feed, continuous cutting | Moderate — 3–5× feed |
Work through the constraints in order: diameter, then depth, then face geometry, then what machines you already own.
Engine blocks, landing gear cylinders, wind/marine shafts — 40+ mm, deep L/D, high volume on a dedicated machine. Maximum feed and the cleanest chip evacuation.
Retrofit the double-tube system instead of buying new iron. Most economical when conventional machines are on the floor and volumes are low-to-moderate.
Castings, forgings, sawn faces, non-round parts — anything a pressure head cannot seal. Only a guide bush or pilot hole is required.
Its larger outer tube gives high torsional stiffness, allowing higher feed rates — a genuine edge for ejector in the ~18–150 mm band, alongside its retrofit flexibility.
Ejector depth is capped by Venturi vacuum (~−0.06 MPa). Deep, straight bores belong on BTA, with counter-rotation for the highest ratios.
Genuinely a coin flip — compare your volume. High volume favors BTA unit cost; retrofit economics favor ejector on existing machines.
STS is more reliable than ejector where chipbreaking is hard — its higher coolant pressure keeps chips moving; the Venturi suction can struggle on gummy steels.
The ejector twin-tube geometry cannot fit — BTA from ~19–20 mm, or gundrill below that. No internal-chip alternative at the small end.
| Myth | Reality |
|---|---|
| “Ejector drilling is a completely separate technology” | It is a Sandvik variation of the BTA system — the same multi-edge head and guide pad family, rerouted through twin tubes. |
| “BTA is only for huge holes” | Practical from ~19–20 mm; ISCAR catalogs single-tube heads from 14.5 mm. |
| “Ejector cannot drill deep holes” | It reaches ~100×D and ~1,000 mm standard (2,000 mm with special connectors) — just not the extreme ratios of dedicated BTA. |
| “Ejector needs less coolant” | False — the Venturi runs on flow rate. Standard practice actually over-feeds 10–80% above the true minimum; starvation is the failure mode. |
| “BTA always needs 1,000+ psi coolant” | 15–100 bar (225–1,450 psi) is typical; the 1,400 psi figure is the ceiling as depth and diameter grow, not the day-one setting. |
| “Ejector equals gundrill productivity” | No — ejector runs 3–5× gundrill feed; BTA runs 5–10×. Both are internal-chip systems, but BTA remains the throughput king. |
| “A flat face is required for deep hole drilling” | Only for BTA. The ejector system’s entire reason to exist is removing that requirement. |